Pressure atomizing fuel nozzle assemblies have been and are used in gas turbine engines. Typically, the nozzle assembly includes an inlet fitting connected to a fuel manifold providing pressurized fuel from a suitable fuel pump. The fuel nozzle assembly is constructed to provide a so-called primary fuel flow for engine start-up and low power operation and a secondary fuel flow for higher power engine requirements. In the past, the primary fuel flow has been controlled by a simple check valve which opens when fuel pressure exceeds a certain relatively low value and remains open during engine operation to supply fuel to the nozzle attached to the assembly. The secondary fuel flow has been controlled by a so-called metering valve which is also pressure-responsive and is designed to open and meter fuel starting at another fuel pressure higher than that opening the check valve. The metering valve supplies additional fuel to the nozzle in preselected relation to changes in fuel pressure.
A pressure atomizing fuel nozzle assembly used in the past includes a secondary fuel metering valve in a main support housing to meter secondary fuel flow to a feed conduit extending through a support strut to a nozzle mounted on the strut. The primary fuel check valve is of the in-line type having a valve head supported atop a relatively small coil spring located in a lateral extension of the support housing. The check valve opens in response to a certain fuel pressure to supply primary fuel to a feed conduit extending through the support strut to the nozzle on the inside of the feed conduit carrying secondary fuel and concentric therewith. In operation, fuel flows into the main support housing upstream of the secondary fuel metering valve and also upstream of the check valve via a bypass to the lateral housing extension. The check valve is operable to admit fuel to the primary fuel passage bypassing the secondary fuel metering valve when fuel pressure reaches a given relatively low value and the secondary fuel metering valve is operable at the higher fuel pressure to admit and meter fuel to the secondary fuel passage located exteriorly of the primary fuel passage in the strut.
Such a fuel nozzle assembly may suffer from certain problems in operation and maintenance under the gas turbine engine conditions to which it will be exposed. For example, a "sticking" problem could be experienced by the primary fuel check valve because of its low spring closing force dictated by its small size, causing fuel leakage and drainage from the fuel manifold past the valve. The check valve controls only the primary fuel flow and there is no means in the nozzle assembly for controlling fuel flow from the inlet to the secondary fuel metering valve. Thus, fuel in the fuel manifold may leak directly past the metering valve if proper sealing action is not achieved. The secondary fuel feed conduit could experience coking as a result of compressor discharge air temperatures heating the secondary fuel flowing through the support strut on the outside of the primary fuel flow. Due to the secondary fuel conduit being concentric around the exterior of the primary fuel conduit and having relatively small passages in a series of standoffs therebetween, unwanted pressure drops may occur in the secondary fuel conduit. Furthermore, there is no provision in the fuel nozzle assembly for washing the fuel filter through which the primary fuel flows to the check valve, and dirt and foreign matter thus may be bothersome. And, no provision is made for providing a cooling action to the support housing and secondary fuel therein against engine heat.